Conventional techniques for melting sulfur often involve mixing crushed, formed or otherwise solid sulfur with liquid sulfur in a tank that has been kept at temperatures above the melting point of sulfur and maintaining the contents of the tank at such temperatures by heating means. Crushed solid sulfur normally originates from solid sulfur lumps and from solid sulfur storage blocks, commonly known as “sulfur vats”; formed solid sulfur usually comes from special industrial operations designed to make specific forms of solid sulfur, such as slate sulfur and sulfur prills, sulfur pellets sulfur pastilles and other such types of granulated sulfur, which are intended for later melting and use as sulfur feed material in various industrial processes. At atmospheric temperatures sulfur is solid; and it remains solid as long as its temperature remains below approximately 240° F.; above this temperature sulfur becomes a fairly fluid liquid; and it remains a relatively low-viscosity fluid until its temperature reaches about 318° F. Above 318° F. sulfur turns very viscous and becomes difficult to pump.
A process for melting sulfur is described in U.S. Pat. No. 3,355,259, of Lipps et al, in which solid sulfur is fed into a tank and mixed with molten sulfur that has been maintained in liquid state at temperatures between 238° F. and 320° F. by the introduction of hot combustion product gases at certain points below the surface of the molten sulfur. This technique has had some commercial applications in the past, but its use is not cost efficient nowadays for various reasons, among them the additional costs required to monitor, process and control the hot combustion product gases in order to maintain the emissions within current environmental discharge requirements. In addition, the combustion product gases introduce contaminants into the liquid sulfur which translate into additional purification costs downstream and/or in the subsequent processing of the molten sulfur product. Certain other known processes for melting sulfur accomplish the melting by providing a melting vessel and introducing steam coils inside the vessel. These processes are able to produce molten sulfur but their overall efficiencies are limited by the limited heat transfer surface area and the size of the vessels that such arrangements entail.
It is an object of the present invention to provide a system and a method for melting sulfur that do not introduce into the molten sulfur any hot combustion gases or any other external sources of heating media, thus avoiding the cost efficiency disadvantages and the contamination problems associated with processes such as the Lipps et al process. It is also an object of this invention to provide a system and a method for the effective melting of crushed or formed solid sulfur that expedite and improve the removal of underflow solids from the tanks where most of the melting takes place and that do not require the introduction of steam coils inside the melting vessels. Another object of the invention is to provide safe sulfur melting methods and systems that may be fabricated, installed and operated at low capital costs, with high throughput rates at high operating efficiencies and low maintenance costs. A further object of the invention is to provide a practical and efficient system and a practical and efficient method for melting solid sulfur that lend themselves to modular fabrication and factory-assembly for easy and cost-effective shipping and on-site assembly. Yet another object of the invention is to provide a system and a method for the effective melting of solid sulfur that allow all of the molten sulfur to be safely contained during unplanned shutdown periods and where all of the vessels and equipment are located above ground, thereby eliminating or minimizing water intrusion, heat losses and other maintenance problems associated with systems and methods that locate vessels or equipment below ground. Still another object of the invention is to provide a system and a method for melting sulfur that allow the flexibility of processing both low and high volumes of solid sulfur feeds without sacrificing either safety or cost effectiveness. An additional object of the invention is to provide a system and a method for the effective melting of solid sulfur that can be industrially fabricated, installed and operated with minimal or no environmental consequences. These and other objects of the invention will become apparent from the descriptions that follow.